Interleukin-15 (IL-15) is a pro-inflammatory cytokine, a glycoprotein of 14-15 kD. Constitutive expression has been reported in various cells and tissues including monocytes and macrophages, fibroblasts, keratinocytes and dendritic cells (Waldmann and Tagaya, 1999; Fehniger and Caligiuri, 2001). The expression is upregulated under inflammatory conditions, as reported for monocytes stimulated with IFN-γ and LPS or by infection with viruses, bacteria or protozoans (Kirman et al., 1998; Waldmann et al., 1998; Waldmann and Tagaya, 1999; Fehniger and Caligiuri, 2001). Furthermore, in chronic inflammatory diseases such as rheumatoid arthritis, locally produced IL-15 is likely to amplify inflammation by the recruitment and activation of synovial T-cells. This IL-15-induced effect has been suggested to play a pivotal role in disease pathogenesis (Kirman et al., 1998; McInnes et al., 1996; McInnes et al., 1997; McInnes and Liew, 1998; Fehniger and Caligiuri, 2001).
In vitro studies have shown that IL-15 shares several biological activities with IL-2, due to shared receptor components. The IL-15 receptor present on T-cells consists of an unique α-chain, IL-15Rα, but shares the β-chain and the γ-chain with IL-2R. As a consequence, both receptors use the same Jak/STAT-signaling elements. However, based on complex regulation and differential expression of IL-2 and IL-15 and their receptors, critical differences in the in vivo functions have been reported (Kirman et al., 1998; Waldmann and Tagaya, 1999; Waldmann et al., 2001). It is also important to note the non-redundant role for IL-15 in natural killer (NK) cell, NK-T cell and intraepithelial lymphocyte development, survival, expansion and function (Kennedy et al., 2000; Liu et al., 2000).
McInnes and coworkers (McInnes et al., 1997; McInnes and Liew, 1998) reported the induction of TNF-α production after IL-15 stimulation in T-cells derived from rheumatoid arthritis patients. Furthermore, peripheral blood T cells activated by IL-15 were shown to induce significant TNF-α production by macrophages via a cell-contact-dependent mechanism. Because of the destructive role of TNF-α in rheumatoid arthritis, inhibition of this cytokine decreases disease activity (Bathon et al., 2000; Klippel, 2000; Lovell et al., 2000; Maini and Taylor, 2000).